This invention relates to encoding of signals and, more particularly, to differential encoding of television signals comprising concatenated frame signals.
FIGS. 1 and 2 describe in general terms the prior art encoder disclosed in U.S. Pat. No. 5,128,756, issued Jul. 7, 1992. In the process carried out in the FIG. 1 arrangement, a frame signal is applied to element 9 where the mean of the signal is subtracted. The output of element 9 is applied to element 11 where a prediction of the frame signal is subtracted. The resulting signal, which in effect is a measure of prediction error, is applied to coder 12 and the output of the coder is transmitted to a destination receiver. The output of the coder is also applied to a corresponding decoder 13 which to the best of its ability reconstitutes the prediction error signal of element 11. Adding the prediction signal to the output of decoder 13, via adder 14, yields a reconstituted frame signal itself (first coded and then decoded) and that signal is stored in memory 15. For the next frame of the input signal the stored frame signal is augmented by displacing, in block 16, signal portions with applied motion vectors and by subtracting the mean signal from the motion displaced frame. This is accomplished in element 17. Lastly, the signal developed by element 17 is multiplied by a leak factor in element 18 to form the prediction signal for the current frame of the input signal.
In a receiver that accepts the coded signals developed and transmitted by coder 12, such as the receiver depicted in FIG. 2, a similar decoding arrangement is found. The received signal is first decoded in element 23 (as in element 13), and adder 24 adds thereto a prediction signal that is developed from a previous frame signal stored in buffer 25. That prediction signal corresponds to the stored frame signal displaced by received motion vectors in element 26, with a received mean subtracted therefrom in element 21 prior to being multiplied by a received leak signal in block 22. The decoded signal and the added prediction signal are augmented in adder 27 by the received mean signal to form a decoded frame signal, and that signal is stored in buffer 25.
The leak factor is introduced in the transmitter in recognition of the fact that the receiver and the transmitter will not always track each other. Aside from noise perturbations, it is clear that a TV receiver freshly tuning into a particular transmitter cannot have data in its buffer that corresponds to the data in the transmitter's storage 15. The leak allows coder 12 to encode a fraction of the input frame signal, and this fraction is communicated to the receiver. Because of the leak factor, the data in the receiver buffer reaches the level of the data in storage 15 within a few frame intervals and is thus synchronized to the transmitter. When the leak factor is 0, no prediction takes place and the receiver is brought into synchronization within a single frame. A somewhat larger leak requires a larger number of frames of the input signal before the data in the receiver's buffer is synchronized with the data in storage 15.
The arrangement of FIG. 1 can operate in analog as well as in digital environments. Some of the current efforts to design an HDTV system for the U.S. are directed to a digital implementation. In the aforementioned U.S. Patent, the thinking was to employ a leak factor that, in accordance with characteristics of the transmitted signal, varies in steps of 1/16, from 0 to 15/16. These steps were selected, in part, because in digital implementations it is very easy to realize division by a power of 2. Each shift of the signal by 1 bit corresponds to a division by 2. Thus, multiplying by 3/16, for example, can be easily accomplished by multiplying by 3 and then shifting the signal by 4 bits.
Alas, this simplicity comes at a price, and the price is loss of the fractional part of the quotient that a proper division may produce. This can be appreciated best, perhaps, from the following example where the leak factor is 15/16 and where multiplication by the 15/16 leak factor is accomplished by shifting the signal by 4 bits and subtracting the shifted signal from the unshifted signal (i.e, evaluating S-(1/16)S, where S is the signal). The structure for carrying out this 15/16 multiplication is illustrated in FIG. 1 by shift circuit 181 followed by subtractor circuit 182. Whereas on first blush the FIG. 1 structure appears to truly effect a 15/16 multiplication, actually it introduces a truncation error because the computations are carried out in integer arithmetic. To illustrate, when the signal S is 128, then (15/16)S is 120; but so is the output for any other signal level between 121 and 133.
Understandably, it is highly desirable to eliminate the artifacts that come about from the truncation error restilting from use of integer arithmetic.